The formation of diaryl phosphide salts from triaryl phosphine and triarylphosphine adducts is an important commercial process. Alkali metal diaryl phosphide salts are used as synthetic intermediates in the synthesis or manufacture of various mono-substituted diaryl phosphines, and complex phosphine ligands for organometallic catalysts as well as phosphine derivatives for various other applications such as phase transfer catalysis, epoxy curing agents, or flame retardants.
There are currently several methods by which this reduction is accomplished. Lithium metal cleavage of triphenyl phosphine to lithium diphenyl phosphide occurs in THF at 25° C. However, lithium diaryl phosphides interact and react with ethereal solvent thus facilitating its decomposition, tend to lead to a loss or decrease in substituted diaryl phosphine product yield and purities, are less reactive due to greater P-M bond covalency (M=Li), and additionally, lithium metal is less desirable due to greater cost and need for argon atmosphere to prevent hazardous lithium nitride formation.
Potassium metal can also be used as the alkali metal reductant alone, without the need for catalyst, to react with triaryl phosphines and its derivatives under typical useful conditions, but again is prone to side reactions, i.e. alkyl halide decomposition that tend to lead to a loss or decrease in substituted diaryl phosphine product yield and purities. Moreover, use of lithium and potassium alkali metals can give rise to significant aryl group scrambling on either the phosphine and phosphide species, i.e. they have low specificity for Ar2PPh, (Ar′)ArPPh, etc. derived from mixed aryl phosphine precursors, which is especially true for lithium metal.
Sodium by itself or with previously reported additives such as amines is reported to insufficiently cleave electron rich phosphines since it is reported to possess a reducing capacity for phosphines that is significantly lower than that of either lithium or potassium. (Schmidt, U.; Kabitzke, K. Markau, K.; Muller, A. Chemische Berichte 1966, 99, 1497). Therefore sodium by itself is not a preferred alkali metal for this use. On the other hand, sodium is a cost effective alternative to lithium and potassium, is easily handled on a large scale, and does not facilitate deleterious phoshine aryl group scrambling. Therefore, from a cost and commercial synthetic perspective, sodium is a more desirable alkali metal reductant.
U.S. Pat. No. 5,866,720 and U.S. Pat. No. 5,777,169 disclose the reduction of triarylphosphine in the presence of molecular hydrogen in an “anhydrous organic liquid diluent.” The organic diluent is paraffinic, cylcoparaffinic, or aromatic hydrocarbon based, preferably THF. Non-alkylated naphthenics and amine catalysts may be present without deleterious effects.
U.S. Pat. No. 5,710,340 discloses a process for forming cycloalkyldiarylphosphines from triarylphosphines using sodium or potassium. THF is the preferred solvent.
Olah and Hehemann, J. Org. Chem., Vol. 42, No 12, 1977, p 2190 describe the reduction of triphenyl phosphine sulfide (Ph3P═S) to triphenyl phosphine (Ph3P) by charging stoichiometric sodium naphthalenide complex. The authors did not contemplate continuing the exposure of the triphenyl phosphine to more sodium naphthalenide or alkali metal (if used in excess relative to naphthalene) to produce further, quantitative conversion to the sodium diphenyl phosphide salt.
Surprisingly, it has now been found that the reducing activity of sodium may be boosted and facilitated by the formation of polycyclic aromatic hydrocarbon (PAH) radical anion reductants to prepare alkali metal diaryl phosphide salts from both electron rich-, deficient-, and mixed triaryl phosphines on acceptable manufacturing timescales.
The cleavage of triaryl phosphines and their relevant derivatives to their corresponding alkali metal diaryl phosphide salts was found to readily occur with sodium metal in tetrahydrofuran at 55° C. in the presence of various dissolving alkali metal polycyclic aromatic hydrocarbons (naphthenic additives). Specifically, the polycyclic aromatic hydrocarbons employed may be of three forms: a perhydrotreated naphthenic oil by-product or distillate of specific identity, a discrete small molecule such as naphthalene or its (per)alkylated and/or heteroatom-containing analog, or a (co)polymer supported form in which the PAH catalytic unit is incorporated either in the main chain or as a pendant group.
Specific examples of active polycyclic aromatic hydrocarbons include perhydrotreated naphthenic oils, naphthalene, [1-methyl]naphthalene, and 1-[N,N-dimethylamino]naphthalene, and/or homo- or copolymers containing the naphthenic unit in any of its indicated substituted derivatives as either a pendant or main chain unit or a combination thereof. Some or all of the PAH catalyst may be reduced to an unreactive x,y-dihydro-derivative that must be separated from the finished product.
Further, with the use of appropriate separation techniques, each of these PAH forms of catalyst can be readily and effectively removed from the process prior to crystallization.